Prior art solar electric power supplies for space satellites use large flat plate arrays of solar cells made from silicon, gallium arsenide, or another semiconductor material. These prior art flat plate solar arrays are generally very expensive due to the large area of semiconductor material required. These prior art solar arrays are generally heavy due to the combined mass of solar cell, cover glass, and backup structure. These prior art arrays are generally bulky and fragile, complicating their packaging for launch into space. These prior art arrays are also complex to deploy on orbit, requiring mechanical means to unfold the arrays and properly position them for operation.
To minimize or overcome these problems, new approaches to solar electric power supplies for space satellites have recently been developed by Fraas and O'Neill, U.S. Pat. No. 5,344,497 and U.S. Pat. No. 5,505,789. The new approaches use Fresnel lenses to collect and focus sunlight onto very high efficiency tandem-junction solar cells. By using a relatively inexpensive Fresnel lens to collect the sunlight and to focus it onto much smaller solar cells, the cost and weight of the cells are dramatically reduced. By using very high efficiency cells, the required array area is minimized, reducing overall system weight and launch volume. The advantages of the new Fresnel lens concentrating solar electric power supply are now being widely recognized, as witnessed by the selection of this approach for the NASA Jet Propulsion Laboratory's New Millennium Deep Space One satellite which was launched Oct. 24, 1998. The Fresnel lens concentrating solar array on Deep Space One provides not only the power for the satellite, but also the power for the electric propulsion system which is propelling the probe to encounters with an asteroid and a comet.
Despite the many advantages of the Fresnel lens concentrating solar array previously invented by O'Neill and Fraas, this array still has shortcomings in need of improvement. Specifically, the Fresnel lens is presently made from a space-qualified, optically clear silicone rubber material (e.g., Dow Corning DC 93-500). This thin rubber lens (e.g., 250 microns thick) must be laminated to a thin (e.g., 80 microns thick) ceria-doped glass superstrate to maintain the required arch shape of the lens assembly. The glass is a structural component, not required for the optical functioning of the lens. Unfortunately, the glass increases the weight, cost, launch volume, and fragility of the lens. Until now, if the glass were not used, the lens would not maintain its shape, even in the zero-gravity environment of space. The presently used glass/silicone Fresnel lens also requires a supporting structure to properly position the lens above the solar cells. This lens support structure adds further weight, cost, and complexity to the solar power system. The presently used glass/silicone Fresnel lens is also not flexible enough to be flattened for compact launch stowage, resulting in a higher than desired launch volume. The presently used glass/silicone Fresnel lens is affected by the difference in thermal expansion coefficients of the glass and silicone layers, causing either stresses or strains in the lens during temperature variations which occur when the satellite moves in and out of the Earth's shadow.
One means of addressing some of the problems associated with the glass arch in the presently used glass/silicone Fresnel lens is to make the polymer lens from a stronger, thicker material, obviating the structural need for the glass arch. Many stronger, thicker polymer lens materials have been evaluated by the present inventor and his colleagues at NASA and in the aerospace industry; these candidate monolithic lens materials include DuPont Teflon.RTM. and Dyneon THV.RTM. and similar fluoropolymer materials from other suppliers. These flexible lens materials offer the possibility of flattening the arched lens in a compact stow position during satellite launch, with mechanical deployment of the arches on orbit in space. The present inventor has worked closely with both NASA and AEC-ABLE Engineering Company on the development of such flexible monolithic polymer lenses. In U.S. Pat. No. 5,496,414, Harvey et al. of AEC-ABLE describe one novel means of stowing and deploying such a monolithic polymer lens. In U.S. Pat. No. 5,578,139, Jones et al. of AEC-ABLE describe another novel means of stowing and deploying such a monolithic polymer lens. However, these prior art lenses must be thick enough and strong enough to be self supporting during ground testing, and therein lies their disadvantage. The lens thickness required to be self-supporting under ground testing is typically 250 microns or more for an 8 cm lens aperture width. Since the density of fluoropolymers is about double the density of the normal silicone rubber lens material, and the total lens thickness is about the same, the flouropolymer lenses weigh about twice as much as the silicone lenses. Thus, even with the added weight of the glass arch superstrate, the old glass/silicone lens is typically lighter than the new monolithic fluoropolymer lens. In addition, the monolithic fluoropolymer lens material does not have the proven successful space flight history and heritage of the silicone lens material.
To overcome these problems with prior art Fresnel lens solar concentrators for space power applications, I have invented a stretched Fresnel lens, which provides all of the benefits of solar concentration, while overcoming the optical efficiency, complexity, cost, weight, launch volume, fragility, and differential thermal expansion problems of the prior art approaches.
Other inventors have proposed many approaches to lightweight solar concentrators for both terrestrial power and space power applications. Indeed, several have proposed stretched membrane concentrators in the past. However, all of these approaches have had substantial shortcomings, as described in more detail the following paragraphs, according to the type of prior art.
Most of the prior art solar concentrators which employ stretched optical elements are reflective concentrators which use mirrored surfaces to focus sunlight. These prior art stretched membrane reflective concentrators are described in the following two paragraphs, which address terrestrial and space reflective concentrators, respectively.
Terrestrial Reflective Stretched Membrane Concentrators: In U.S. Pat. No. 3,924,604, Anderson describes a terrestrial, stretched membrane, flat reflective heliostat for focussing sunlight onto a central receiver on top of a tall tower. Similarly, in U.S. Pat. No. 4,046,462, Miller et al. describe a terrestrial, stretched membrane, three-dimensionally curved reflective solar concentrator. Likewise, in U.S. Pat. No. 4,134,387, Tornstrum describes a terrestrial, stretched membrane, segmented flat reflective concentrator with replaceable reflective material on reels. In similar vein, in U.S. Pat. No. 4,237,864, Kravitz describes a terrestrial, stretched membrane, parabolic drape reflective concentrator. Similarly, in U.S. Pat. No. 4,487,196, Murphy describes a terrestrial, stretched membrane, three-dimensionally curved reflective concentrator. In like vein, in U.S. Pat. No. 4,493,313, Eaton describes a terrestrial, stretched membrane, parabolic trough reflective concentrator. Very similarly, in U.S. Pat. No. 4,596,238, Bronstein describes another terrestrial, stretched membrane, parabolic trough reflective concentrator. Likewise, in U.S. Pat. No. 4,744,644, Kleinwuchter et al. describe a terrestrial, stretched membrane, parabolic dish reflective concentrator. In like vein, in U.S. Pat. No. 5,210,654, Williams describes a terrestrial, stretched membrane, three-dimensionally curved reflective concentrator.
Space Reflective Stretched Membrane Concentrators: In U.S. Pat. No. 4,719,903, Powell describes a space, stretched membrane, parabolic trough reflective concentrator. Similarly, in U.S. Pat. No. 5,202,689, Bussard et al. describe a space, stretched membrane, peripherally supported reflective concentrator. Likewise, in U.S. Pat. No. 5,660,644, Clemens describes a space, stretched membrane, parabolic trough reflective concentrator. In like vein, in U.S. Pat. No. 5,865,905, Clemens describes a space, stretched membrane, parabolic trough reflective concentrator, with replaceable reflective material on reels, similar to the terrestrial system of Tornstrum in the previous paragraph.
All of these prior art stretched membrane reflective concentrators, whether meant for terrestrial or space application, suffer from an overwhelming, 100-fold disadvantage in shape error tolerance, compared to my new stretched lens refractive concentrator. All reflective concentrators, whether stretched membrane or rigid in construction, require a much higher degree of surface accuracy than an optimized refractive concentrator, as discussed by O'Neill in Chapter 10 of the textbook, Solar Cells and their Applications, published by John Wiley in 1995. An optimized refractive concentrator corresponds to the Fresnel lens configuration taught by O'Neill in U.S. Pat. No. 4,069,812. Since reflective concentrators need more than 100 times better shape accuracy than an optimized refractive concentrator, it will be more difficult, expensive, and risky to implement a space power system using the prior art reflective stretched membrane solar concentrators, than one using my new refractive stretched membrane Fresnel lens solar concentrator.
A few prior art solar concentrators have proposed thin, non-self-supporting Fresnel lenses stretched between rigid structural elements. The present inventor has worked with 3M in the terrestrial solar concentrator area and with Harris Corporation in the space solar concentrator area. In U.S. Pat. No. 4,848,319, Appeldorn of 3M teaches a thin linear Fresnel lens stretched between longitudinal structural elements, thereby creating a polygonal lens in cross section, for terrestrial applications. Similarly, Grayson et al. of Harris teach a more complex parquet of thin, flat, Fresnel lens elements, stretched between radial elements, to form an umbrella-like dome lens for space applications. Both of these prior art stretched lens approaches have many deficiencies, compared to my new design. The approach of Appeldorn provides a poor straight-line-segment approximation of the desired, ideal curvilinear arch lens shape. Furthermore, the longitudinal structural elements of Appeldorn block a large fraction of the available lens aperture, thereby preventing sunlight from entering the concentrator. Thus, Appeldorn's approach fails to achieve either the desired lens shape or the high optical efficiency needed for good solar concentrator performance. Similarly, the approach of Grayson et al. is likewise flawed, providing flat lens elements instead of the desired curved lens elements, and suffering even more light blockage than Appeldorn's approach, due to the complex umbrella-like support structure.
In addition, both Appeldorn and Grayson et al. teach a relatively complex structure, which will be both heavy and costly. For space applications, a lightweight structure is crucial, since the present cost of launching 1 kg of satellite mass into geostationary orbit is between $50,000 and $100,000.
Still further, both Appeldorn and Grayson et al. fail to adequately address one critical problem with stretched lens concentrators: differential thermal expansion between the lens material and the support structure, especially in the direction of greatest linear dimension. The presently used silicone lens material expands and contracts at a relatively gigantic rate with temperature: more than 300 parts per million per degree Centigrade (C.). Furthermore, solar arrays on geostationary (GEO) satellites undergo huge temperature swings from the warm, illuminated portions of the orbit, to the cold, dark, eclipsed portions of the orbit, when the satellite is in the Earth's shadow. A typical array must be able to survive at least 1,500 thermal cycles from -180C. to +80C. array temperature. An unrestrained 30 cm (1 foot) long lens will expand and contract more than 2 cm (8%) in length during this temperature excursion. In contrast, a typical graphite/epoxy space structure will expand and contract several hundred times more slowly with temperature than the lens material. Thus, the differential thermal expansion problem must be addressed for an acceptable stretched lens space solar concentrator, but the prior art does not teach a solution to this problem.
In contrast to the prior art, my new invention uses one-dimensional lengthwise tension to support the thin lens material in the space environment. This novel approach enables the lens to maintain an ideal, arched, curvilinear shape, with absolutely no aperture blockage over the full stretched length of the lens. In addition, my invention allows the thin, ultralightweight lens to be folded flat against the photovoltaic receiver/waste heat radiator assembly, for minimal launch volume. Once on orbit in space, my lens can be readily deployed by allowing the end arch structures to pop up into place, thereby lightly tensioning the lens in one direction. Furthermore, my invention is compatible with the optimized refractive concentrator approach taught by O'Neill, U.S. Pat. No. 4,069,812. Since this optimized lens maximizes both optical efficiency and shape error tolerance, it is ideally suited for a stretched lens concentrator application. Still furthermore, by maintaining a small lengthwise stretching force on the lens, either with springs or flexible structure, differential thermal expansion and contraction of the lens relative to the receiver/radiator structure is easily accommodated.
The dramatic extent of the improvement provided by my invention can be appreciated by comparing the power-o-mass ratio, also called specific power, of existing, state-of-the-art space solar arrays to future arrays employing my new invention. The current SCARLET array, which uses glass/silicone lenses, has an outstanding specific power of about 50 Watts per kilogram. But an array using my new stretched lens concentrator should extend this critical system performance index to 300 Watts per kilogram, a six-fold gain. This extraordinary prediction is based on measured mass and performance values for functional prototypes of my invention. These prototypes provided a measured 92% net optical efficiency and weighed less than 1 kilogram per square meter of lens aperture area, including not only the lens weight, but also the spring-loaded pop-up end arches, the waste heat radiator, and simulated solar cells.
In summary, my stretched Fresnel lens invention overcomes many prior art problems in space solar concentrator arrays.